Network Working Group P. Hoffman
Internet-Draft ICANN
Obsoletes: 7719 (if approved) A. Sullivan
Updates: 2308 (if approved) Oracle
Intended status: Best Current Practice K. Fujiwara
Expires: January 17, 2019 JPRS
July 16, 2018
DNS Terminologydraft-ietf-dnsop-terminology-bis-11
Abstract
The domain name system (DNS) is defined in literally dozens of
different RFCs. The terminology used by implementers and developers
of DNS protocols, and by operators of DNS systems, has sometimes
changed in the decades since the DNS was first defined. This
document gives current definitions for many of the terms used in the
DNS in a single document.
This document will be the successor to RFC 7719, and thus will
obsolete RFC 7719. It will also update RFC 2308.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
Hoffman, et al. Expires January 17, 2019 [Page 1]

Internet-Draft DNS Terminology July 2018
Most of the definitions here are the consensus definition of the DNS
community -- both protocol developers and operators. Some of the
definitions differ from earlier RFCs, and those differences are
noted. In this document, where the consensus definition is the same
as the one in an RFC, that RFC is quoted. Where the consensus
definition has changed somewhat, the RFC is mentioned but the new
stand-alone definition is given. See Appendix A for a list of the
definitions that this document updates.
It is important to note that, during the development of this
document, it became clear that some DNS-related terms are interpreted
quite differently by different DNS experts. Further, some terms that
are defined in early DNS RFCs now have definitions that are generally
agreed to, but that are different from the original definitions.
Therefore, this document is a substantial revision to [RFC7719].
The terms are organized loosely by topic. Some definitions are for
new terms for things that are commonly talked about in the DNS
community but that never had terms defined for them.
Other organizations sometimes define DNS-related terms their own way.
For example, the W3C defines "domain" at
<https://url.spec.whatwg.org/>. The Root Server System Advisory
Committee (RSSAC) has a good lexicon [RSSAC026].
Note that there is no single consistent definition of "the DNS". It
can be considered to be some combination of the following: a commonly
used naming scheme for objects on the Internet; a distributed
database representing the names and certain properties of these
objects; an architecture providing distributed maintenance,
resilience, and loose coherency for this database; and a simple
query-response protocol (as mentioned below) implementing this
architecture. Section 2 defines "global DNS" and "private DNS" as a
way to deal with these differing definitions.
Capitalization in DNS terms is often inconsistent among RFCs and
various DNS practitioners. The capitalization used in this document
is a best guess at current practices, and is not meant to indicate
that other capitalization styles are wrong or archaic. In some
cases, multiple styles of capitalization are used for the same term
due to quoting from different RFCs.
2. Names
Naming system: A naming system associates names with data. Naming
systems have many significant facets that help differentiate them.
Some commonly-identified facets include:
Hoffman, et al. Expires January 17, 2019 [Page 3]

Internet-Draft DNS Terminology July 2018
* Composition of names
* Format of names
* Administration of names
* Types of data that can be associated with names
* Types of metadata for names
* Protocol for getting data from a name
* Context for resolving a name
Note that this list is a small subset of facets that people have
identified over time for naming systems, and the IETF has yet to
agree on a good set of facets that can be used to compare naming
systems. For example, other facets might include "protocol to
update data in a name", "privacy of names", and "privacy of data
associated with names", but those are not as well-defined as the
ones listed above. The list here is chosen because it helps
describe the DNS and naming systems similar to the DNS.
Domain name: An ordered list of one or more labels.
Note that this is a definition independent of the DNS RFCs, and
the definition here also applies to systems other than the DNS.
[RFC1034] defines the "domain name space" using mathematical trees
and their nodes in graph theory, and this definition has the same
practical result as the definition here. Using graph theory, a
domain name is a list of labels identifying a portion along one
edge of a directed acyclic graph. A domain name can be relative
to parts of the tree, or it can be fully qualified (in which case,
it begins at the common root of the graph).
Also note that different IETF and non-IETF documents have used the
term "domain name" in many different ways. It is common for
earlier documents to use "domain name" to mean "names that match
the syntax in [RFC1035]", but possibly with additional rules such
as "and are, or will be, resolvable in the global DNS" or "but
only using the presentation format".
Label: An ordered list of zero or more octets and which makes up a
portion of a domain name. Using graph theory, a label identifies
one node in a portion of the graph of all possible domain names.
Global DNS: Using the short set of facets listed in "Naming system",
the global DNS can be defined as follows. Most of the rules here
Hoffman, et al. Expires January 17, 2019 [Page 4]

Internet-Draft DNS Terminology July 2018
come from [RFC1034] and [RFC1035], although the term "global DNS"
has not been defined before now.
Composition of names -- A name in the global DNS has one or more
labels. The length of each label is between 0 and 63 octets
inclusive. In a fully-qualified domain name, the first label in
the ordered list is 0 octets long; it is the only label whose
length may be 0 octets, and it is called the "root" or "root
label". A domain name in the global DNS has a maximum total
length of 255 octets in the wire format; the root represents one
octet for this calculation. (Multicast DNS [RFC6762] allows names
up to 255 bytes plus a terminating zero byte based on a different
interpretation of RFC 1035 and what is included in the 255
octets.)
Format of names -- Names in the global DNS are domain names.
There are three formats: wire format, presentation format, and
common display.
The basic wire format for names in the global DNS is a list of
labels ordered by decreasing distance from the root, with the root
label last. Each label is preceded by a length octet. [RFC1035]
also defines a compression scheme that modifies this format.
The presentation format for names in the global DNS is a list of
labels ordered by decreasing distance from the root, encoded as
ASCII, with a "." character between each label. In presentation
format, a fully-qualified domain name includes the root label and
the associated separator dot. For example, in presentation
format, a fully-qualified domain name with two non-root labels is
always shown as "example.tld." instead of "example.tld".
[RFC1035] defines a method for showing octets that do not display
in ASCII.
The common display format is used in applications and free text.
It is the same as the presentation format, but showing the root
label and the "." before it is optional and is rarely done. For
example, in common display format, a fully-qualified domain name
with two non-root labels is usually shown as "example.tld" instead
of "example.tld.". Names in the common display format are
normally written such that the directionality of the writing
system presents labels by decreasing distance from the root (so,
in both English and C the root or TLD label in the ordered list is
right-most; but in Arabic it may be left-most, depending on local
conventions).
Administration of names -- Administration is specified by
delegation (see the definition of "delegation" in Section 7).
Hoffman, et al. Expires January 17, 2019 [Page 5]

Internet-Draft DNS Terminology July 2018
Policies for administration of the root zone in the global DNS are
determined by the names operational community, which convenes
itself in the Internet Corporation for Assigned Names and Numbers
(ICANN). The names operational community selects the IANA
Functions Operator for the global DNS root zone. At the time this
document is published, that operator is Public Technical
Identifiers (PTI). The name servers that serve the root zone are
provided by independent root operators. Other zones in the global
DNS have their own policies for administration.
Types of data that can be associated with names -- A name can have
zero or more resource records associated with it. There are
numerous types of resource records with unique data structures
defined in many different RFCs and in the IANA registry at
[IANA_Resource_Registry].
Types of metadata for names -- Any name that is published in the
DNS appears as a set of resource records (see the definition of
"RRset" in Section 5). Some names do not themselves have data
associated with them in the DNS, but "appear" in the DNS anyway
because they form part of a longer name that does have data
associated with it (see the definition of "empty non-terminals" in
Section 7).
Protocol for getting data from a name -- The protocol described in
[RFC1035].
Context for resolving a name -- The global DNS root zone
distributed by PTI.
Private DNS: Names that use the protocol described in [RFC1035] but
that do not rely on the global DNS root zone, or names that are
otherwise not generally available on the Internet but are using
the protocol described in [RFC1035]. A system can use both the
global DNS and one or more private DNS systems; for example, see
"Split DNS" in Section 6.
Note that domain names that do not appear in the DNS, and that are
intended never to be looked up using the DNS protocol, are not
part of the global DNS or a private DNS even though they are
domain names.
Multicast DNS: "Multicast DNS (mDNS) provides the ability to perform
DNS-like operations on the local link in the absence of any
conventional Unicast DNS server. In addition, Multicast DNS
designates a portion of the DNS namespace to be free for local
use, without the need to pay any annual fee, and without the need
to set up delegations or otherwise configure a conventional DNS
Hoffman, et al. Expires January 17, 2019 [Page 6]

Internet-Draft DNS Terminology July 2018
server to answer for those names." (Quoted from [RFC6762],
Abstract) Although it uses a compatible wire format, mDNS is
strictly speaking a different protocol than DNS. Also, where the
above quote says "a portion of the DNS namespace", it would be
clearer to say "a portion of the domain name space" The names in
mDNS are not intended to be looked up in the DNS.
Locally served DNS zone: A locally served DNS zone is a special case
of private DNS. Names are resolved using the DNS protocol in a
local context. [RFC6303] defines subdomains of IN-ADDR.ARPA that
are locally served zones. Resolution of names through locally
served zones may result in ambiguous results. For example, the
same name may resolve to different results in different locally
served DNS zone contexts. The context through which a locally
served zone may be explicit, for example, as defined in [RFC6303],
or implicit, as defined by local DNS administration and not known
to the resolution client.
Fully qualified domain name (FQDN): This is often just a clear way
of saying the same thing as "domain name of a node", as outlined
above. However, the term is ambiguous. Strictly speaking, a
fully qualified domain name would include every label, including
the zero-length label of the root: such a name would be written
"www.example.net." (note the terminating dot). But because every
name eventually shares the common root, names are often written
relative to the root (such as "www.example.net") and are still
called "fully qualified". This term first appeared in [RFC0819].
In this document, names are often written relative to the root.
The need for the term "fully qualified domain name" comes from the
existence of partially qualified domain names, which are names
where one or more of the earliest labels in the ordered list are
omitted (for example, a name of "www" derived from
"www.example.net"). Such relative names are understood only by
context.
Host name: This term and its equivalent, "hostname", have been
widely used but are not defined in [RFC1034], [RFC1035],
[RFC1123], or [RFC2181]. The DNS was originally deployed into the
Host Tables environment as outlined in [RFC0952], and it is likely
that the term followed informally from the definition there. Over
time, the definition seems to have shifted. "Host name" is often
meant to be a domain name that follows the rules in Section 3.5 of
[RFC1034], the "preferred name syntax" (that is, every character
in each label is a letter, a digit, or a hyphen). Note that any
label in a domain name can contain any octet value; hostnames are
generally considered to be domain names where every label follows
the rules in the "preferred name syntax", with the amendment that
Hoffman, et al. Expires January 17, 2019 [Page 7]

Internet-Draft DNS Terminology July 2018
labels can start with ASCII digits (this amendment comes from
Section 2.1 of [RFC1123]).
People also sometimes use the term hostname to refer to just the
first label of an FQDN, such as "printer" in
"printer.admin.example.com". (Sometimes this is formalized in
configuration in operating systems.) In addition, people
sometimes use this term to describe any name that refers to a
machine, and those might include labels that do not conform to the
"preferred name syntax".
TLD: A Top-Level Domain, meaning a zone that is one layer below the
root, such as "com" or "jp". There is nothing special, from the
point of view of the DNS, about TLDs. Most of them are also
delegation-centric zones (defined in Section 7, and there are
significant policy issues around their operation. TLDs are often
divided into sub-groups such as Country Code Top-Level Domains
(ccTLDs), Generic Top-Level Domains (gTLDs), and others; the
division is a matter of policy, and beyond the scope of this
document.
IDN: The common abbreviation for "Internationalized Domain Name".
The IDNA protocol is the standard mechanism for handling domain
names with non-ASCII characters in applications in the DNS. The
current standard, normally called "IDNA2008", is defined in
[RFC5890], [RFC5891], [RFC5892], [RFC5893], and [RFC5894]. These
documents define many IDN-specific terms such as "LDH label",
"A-label", and "U-label". [RFC6365] defines more terms that
relate to internationalization (some of which relate to IDNs), and
[RFC6055] has a much more extensive discussion of IDNs, including
some new terminology.
Subdomain: "A domain is a subdomain of another domain if it is
contained within that domain. This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's
name." (Quoted from [RFC1034], Section 3.1). For example, in the
host name "nnn.mmm.example.com", both "mmm.example.com" and
"nnn.mmm.example.com" are subdomains of "example.com".
Alias: The owner of a CNAME resource record, or a subdomain of the
owner of a DNAME resource record. See also "canonical name".
Canonical name: A CNAME resource record "identifies its owner name
as an alias, and specifies the corresponding canonical name in the
RDATA section of the RR." (Quoted from [RFC1034], Section 3.6.2)
This usage of the word "canonical" is related to the mathematical
concept of "canonical form".
Hoffman, et al. Expires January 17, 2019 [Page 8]

Internet-Draft DNS Terminology July 2018
CNAME: "It is traditional to refer to the owner of a CNAME record as
'a CNAME'. This is unfortunate, as 'CNAME' is an abbreviation of
'canonical name', and the owner of a CNAME record is an alias, not
a canonical name." (Quoted from [RFC2181], Section 10.1.1)
3. DNS Response Codes
Some of response codes that are defined in [RFC1035] have acquired
their own shorthand names. All of the RCODEs are listed at
[IANA_Resource_Registry], although that site uses mixed-case
capitalization, while most documents use all-caps. Some of the
common names are described here, but the official list is in the IANA
registry.
NOERROR: "No error condition" (Quoted from [RFC1035],
Section 4.1.1.)
FORMERR: "Format error - The name server was unable to interpret the
query." (Quoted from [RFC1035], Section 4.1.1.)
SERVFAIL: "Server failure - The name server was unable to process
this query due to a problem with the name server." (Quoted from
[RFC1035], Section 4.1.1.)
NXDOMAIN: "Name Error - This code signifies that the domain name
referenced in the query does not exist." (Quoted from [RFC1035],
Section 4.1.1.) [RFC2308] established NXDOMAIN as a synonym for
Name Error.
NOTIMP: "Not Implemented - The name server does not support the
requested kind of query." (Quoted from [RFC1035], Section 4.1.1.)
REFUSED: "Refused - The name server refuses to perform the specified
operation for policy reasons. For example, a name server may not
wish to provide the information to the particular requester, or a
name server may not wish to perform a particular operation (e.g.,
zone transfer) for particular data." (Quoted from [RFC1035],
Section 4.1.1.)
NODATA: "A pseudo RCODE which indicates that the name is valid for
the given class, but there are no records of the given type. A
NODATA response has to be inferred from the answer." (Quoted from
[RFC2308], Section 1.) "NODATA is indicated by an answer with the
RCODE set to NOERROR and no relevant answers in the answer
section. The authority section will contain an SOA record, or
there will be no NS records there." (Quoted from [RFC2308],
Section 2.2.) Note that referrals have a similar format to NODATA
replies; [RFC2308] explains how to distinguish them.
Hoffman, et al. Expires January 17, 2019 [Page 9]

Internet-Draft DNS Terminology July 2018
The term "NXRRSET" is sometimes used as a synonym for NODATA.
However, this is a mistake, given that NXRRSET is a specific error
code defined in [RFC2136].
Negative response: A response that indicates that a particular RRset
does not exist, or whose RCODE indicates the nameserver cannot
answer. Sections 2 and 7 of [RFC2308] describe the types of
negative responses in detail.
4. DNS Transactions
The header of a DNS message is its first 12 octets. Many of the
fields and flags in the header diagram in Sections 4.1.1 through
4.1.3 of [RFC1035] are referred to by their names in that diagram.
For example, the response codes are called "RCODEs", the data for a
record is called the "RDATA", and the authoritative answer bit is
often called "the AA flag" or "the AA bit".
QNAME: The most commonly-used rough definition is that the QNAME is
a field in the Question section of a query. "A standard query
specifies a target domain name (QNAME), query type (QTYPE), and
query class (QCLASS) and asks for RRs which match." (Quoted from
[RFC1034], Section 3.7.1.). Strictly speaking, the definition
comes from [RFC1035], Section 4.1.2, where the QNAME is defined in
respect of the Question Section. This definition appears to be
applied consistently: the discussion of inverse queries in section6.4 refers to the "owner name of the query RR and its TTL",
because inverse queries populate the Answer Section and leave the
Question Section empty. (Inverse queries are deprecated in
[RFC3425], and so relevant definitions do not appear in this
document.)
[RFC2308], however, has an alternate definition that puts the
QNAME in the answer (or series of answers) instead of the query.
It defines QNAME as: "...the name in the query section of an
answer, or where this resolves to a CNAME, or CNAME chain, the
data field of the last CNAME. The last CNAME in this sense is
that which contains a value which does not resolve to another
CNAME." This definition has a certain internal logic, because of
the way CNAME substitution works and the definition of CNAME. If
a name server does not find an RRset that matches a query, but it
finds the same name in the same class with a CNAME record, then
the name server "includes the CNAME record in the response and
restarts the query at the domain name specified in the data field
of the CNAME record." ([RFC1034] Section 3.6.2). This is made
explicit in the resolution algorithm outlined in Section 4.3.2 of
[RFC1034], which says to "change QNAME to the canonical name in
the CNAME RR, and go back to step 1" in the case of a CNAME RR.
Hoffman, et al. Expires January 17, 2019 [Page 10]

Internet-Draft DNS Terminology July 2018
Since a CNAME record explicitly declares that the owner name is
canonically named what is in the RDATA, then there is a way to
view the new name (i.e. the name that was in the RDATA of the
CNAME RR) as also being the QNAME.
This creates a kind of confusion, however, because the answer to a
query that results in CNAME processing contains in the echoed
Question Section one QNAME (the name in the original query), and a
second QNAME that is in the data field of the last CNAME. The
confusion comes from the iterative/recursive mode of resolution,
which finally returns an answer that need not actually have the
same owner name as the QNAME contained in the original query.
To address this potential confusion, it is helpful to distinguish
between three meanings:
* QNAME (original): The name actually sent in the Question
Section in the original query, which is always echoed in the
(final) reply in the Question Section when the QR bit is set to
1.
* QNAME (effective): A name actually resolved, which is either
the name originally queried, or a name received in a CNAME
chain response.
* QNAME (final): The name actually resolved, which is either the
name actually queried or else the last name in a CNAME chain
response.
Note that, because the definition in [RFC2308] is actually for a
different concept than what was in [RFC1034], it would have be
better if [RFC2308] had used a different name for that concept.
In general use today, QNAME almost always means what is defined
above as "QNAME (original)".
Referrals: A type of response in which a server, signaling that it
is not (completely) authoritative for an answer, provides the
querying resolver with an alternative place to send its query.
Referrals can be partial.
A referral arises when a server is not performing recursive
service while answering a query. It appears in step 3(b) of the
algorithm in [RFC1034], Section 4.3.2.
There are two types of referral response. The first is a downward
referral (sometimes described as "delegation response"), where the
server is authoritative for some portion of the QNAME. The
authority section RRset's RDATA contains the name servers
Hoffman, et al. Expires January 17, 2019 [Page 11]

Internet-Draft DNS Terminology July 2018
specified at the referred-to zone cut. In normal DNS operation,
this kind of response is required in order to find names beneath a
delegation. The bare use of "referral" means this kind of
referral, and many people believe that this is the only legitimate
kind of referral in the DNS.
The second is an upward referral (sometimes described as "root
referral"), where the server is not authoritative for any portion
of the QNAME. When this happens, the referred-to zone in the
authority section is usually the root zone (.). In normal DNS
operation, this kind of response is not required for resolution or
for correctly answering any query. There is no requirement that
any server send upward referrals. Some people regard upward
referrals as a sign of a misconfiguration or error. Upward
referrals always need some sort of qualifier (such as "upward" or
"root"), and are never identified by the bare word "referral".
A response that has only a referral contains an empty answer
section. It contains the NS RRset for the referred-to zone in the
authority section. It may contain RRs that provide addresses in
the additional section. The AA bit is clear.
In the case where the query matches an alias, and the server is
not authoritative for the target of the alias but it is
authoritative for some name above the target of the alias, the
resolution algorithm will produce a response that contains both
the authoritative answer for the alias, and also a referral. Such
a partial answer and referral response has data in the answer
section. It has the NS RRset for the referred-to zone in the
authority section. It may contain RRs that provide addresses in
the additional section. The AA bit is set, because the first name
in the answer section matches the QNAME and the server is
authoritative for that answer (see [RFC1035], Section 4.1.1).
5. Resource Records
RR: An acronym for resource record. ([RFC1034], Section 3.6.)
RRset: A set of resource records with the same label, class and
type, but with different data. (Definition from [RFC2181]) Also
spelled RRSet in some documents. As a clarification, "same label"
in this definition means "same owner name". In addition,
[RFC2181] states that "the TTLs of all RRs in an RRSet must be the
same".
Note that RRSIG resource records do not match this definition.
[RFC4035] says: "An RRset MAY have multiple RRSIG RRs associated
with it. Note that as RRSIG RRs are closely tied to the RRsets
Hoffman, et al. Expires January 17, 2019 [Page 12]

Internet-Draft DNS Terminology July 2018
whose signatures they contain, RRSIG RRs, unlike all other DNS RR
types, do not form RRsets. In particular, the TTL values among
RRSIG RRs with a common owner name do not follow the RRset rules
described in [RFC2181]."
Master file: "Master files are text files that contain RRs in text
form. Since the contents of a zone can be expressed in the form
of a list of RRs a master file is most often used to define a
zone, though it can be used to list a cache's contents."
([RFC1035], Section 5.) Master files are sometimes called "zone
files".
Presentation format: The text format used in master files. This
format is shown but not formally defined in [RFC1034] and
[RFC1035]. The term "presentation format" first appears in
[RFC4034].
EDNS: The extension mechanisms for DNS, defined in [RFC6891].
Sometimes called "EDNS0" or "EDNS(0)" to indicate the version
number. EDNS allows DNS clients and servers to specify message
sizes larger than the original 512 octet limit, to expand the
response code space, and potentially to carry additional options
that affect the handling of a DNS query.
OPT: A pseudo-RR (sometimes called a "meta-RR") that is used only to
contain control information pertaining to the question-and-answer
sequence of a specific transaction. (Definition from [RFC6891],
Section 6.1.1) It is used by EDNS.
Owner: The domain name where a RR is found ([RFC1034], Section 3.6).
Often appears in the term "owner name".
SOA field names: DNS documents, including the definitions here,
often refer to the fields in the RDATA of an SOA resource record
by field name. Those fields are defined in Section 3.3.13 of
[RFC1035]. The names (in the order they appear in the SOA RDATA)
are MNAME, RNAME, SERIAL, REFRESH, RETRY, EXPIRE, and MINIMUM.
Note that the meaning of MINIMUM field is updated in Section 4 of
[RFC2308]; the new definition is that the MINIMUM field is only
"the TTL to be used for negative responses". This document tends
to use field names instead of terms that describe the fields.
TTL: The maximum "time to live" of a resource record. "A TTL value
is an unsigned number, with a minimum value of 0, and a maximum
value of 2147483647. That is, a maximum of 2^31 - 1. When
transmitted, the TTL is encoded in the less significant 31 bits of
the 32 bit TTL field, with the most significant, or sign, bit set
to zero." (Quoted from [RFC2181], Section 8) (Note that [RFC1035]
Hoffman, et al. Expires January 17, 2019 [Page 13]

Internet-Draft DNS Terminology July 2018
erroneously stated that this is a signed integer; that was fixed
by [RFC2181].)
The TTL "specifies the time interval that the resource record may
be cached before the source of the information should again be
consulted". (Quoted from [RFC1035], Section 3.2.1) Also: "the
time interval (in seconds) that the resource record may be cached
before it should be discarded". (Quoted from [RFC1035],
Section 4.1.3). Despite being defined for a resource record, the
TTL of every resource record in an RRset is required to be the
same ([RFC2181], Section 5.2).
The reason that the TTL is the maximum time to live is that a
cache operator might decide to shorten the time to live for
operational purposes, such as if there is a policy to disallow TTL
values over a certain number. Some servers are known to ignore
the TTL on some RRsets (such as when the authoritative data has a
very short TTL) even though this is against the advice in RFC1035. An RRset can be flushed from the cache before the end of
the TTL interval, at which point the value of the TTL becomes
unknown because the RRset with which it was associated no longer
exists.
There is also the concept of a "default TTL" for a zone, which can
be a configuration parameter in the server software. This is
often expressed by a default for the entire server, and a default
for a zone using the $TTL directive in a zone file. The $TTL
directive was added to the master file format by [RFC2308].
Class independent: A resource record type whose syntax and semantics
are the same for every DNS class. A resource record type that is
not class independent has different meanings depending on the DNS
class of the record, or the meaning is undefined for some class.
Most resorece record types are defined for class 1 (IN, the
Internet), but many are undefined for other classes.
Address records: Records whose type is A or AAAA. [RFC2181]
informally defines these as "(A, AAAA, etc)". Note that new types
of address records could be defined in the future.
6. DNS Servers and Clients
This section defines the terms used for the systems that act as DNS
clients, DNS servers, or both. In the RFCs, DNS servers are
sometimes called "name servers", "nameservers", or just "servers".
There is no formal definition of DNS server, but the RFCs generally
assume that it is an Internet server that listens for queries and
Hoffman, et al. Expires January 17, 2019 [Page 14]

Internet-Draft DNS Terminology July 2018
sends responses using the DNS protocol defined in [RFC1035] and its
successors.
It is important to note that the terms "DNS server" and "name server"
require context in order to understand the services being provided.
Both authoritative servers and recursive resolvers are often called
"DNS servers" and "name servers" even though they serve different
roles (but may be part of the same software package).
For terminology specific to the public DNS root server system, see
[RSSAC026]. That document defines terms such as "root server", "root
server operator", and terms that are specific to the way that the
root zone of the public DNS is served.
Resolver: A program "that extract[s] information from name servers
in response to client requests." (Quoted from [RFC1034],
Section 2.4) A resolver performs queries for a name, type, and
class, and receives answers. The logical function is called
"resolution". In practice, the term is usually referring to some
specific type of resolver (some of which are defined below), and
understanding the use of the term depends on understanding the
context.
A related term is "resolve", which is not formally defined in
[RFC1034] or [RFC1035]. An imputed definition might be "asking a
question that consists of a domain name, class, and type, and
receiving some sort of answer". Similarly, an imputed definition
of "resolution" might be "the answer received from resolving".
Stub resolver: A resolver that cannot perform all resolution itself.
Stub resolvers generally depend on a recursive resolver to
undertake the actual resolution function. Stub resolvers are
discussed but never fully defined in Section 5.3.1 of [RFC1034].
They are fully defined in Section 6.1.3.1 of [RFC1123].
Iterative mode: A resolution mode of a server that receives DNS
queries and responds with a referral to another server.
Section 2.3 of [RFC1034] describes this as "The server refers the
client to another server and lets the client pursue the query". A
resolver that works in iterative mode is sometimes called an
"iterative resolver". See also "iterative resolution" later in
this section.
Recursive mode: A resolution mode of a server that receives DNS
queries and either responds to those queries from a local cache or
sends queries to other servers in order to get the final answers
to the original queries. Section 2.3 of [RFC1034] describes this
as "The first server pursues the query for the client at another
Hoffman, et al. Expires January 17, 2019 [Page 15]

Internet-Draft DNS Terminology July 2018
server". Section 4.3.1 of of [RFC1034] says "in [recursive] mode
the name server acts in the role of a resolver and returns either
an error or the answer, but never referrals." That same section
also says "The recursive mode occurs when a query with RD set
arrives at a server which is willing to provide recursive service;
the client can verify that recursive mode was used by checking
that both RA and RD are set in the reply."
A server operating in recursive mode may be thought of as having a
name server side (which is what answers the query) and a resolver
side (which performs the resolution function). Systems operating
in this mode are commonly called "recursive servers". Sometimes
they are called "recursive resolvers". While strictly the
difference between these is that one of them sends queries to
another recursive server and the other does not, in practice it is
not possible to know in advance whether the server that one is
querying will also perform recursion; both terms can be observed
in use interchangeably.
Recursive resolver: A resolver that acts in recursive mode. In
general, a recursive resolver is expected to cache the answers it
receives (which would make it a full-service resolver), but some
recursive resolvers might not cache.
[RFC4697] tried to differentiate between a recursive resolver and
an iterative resolver.
Recursive query: A query with the Recursion Desired (RD) bit set to
1 in the header. (See Section 4.1.1 of [RFC1035].) If recursive
service is available and is requested by the RD bit in the query,
the server uses its resolver to answer the query. (See
Section 4.3.2 of [RFC1035].)
Non-recursive query: A query with the Recursion Desired (RD) bit set
to 0 in the header. A server can answer non-recursive queries
using only local information: the response contains either an
error, the answer, or a referral to some other server "closer" to
the answer. (See Section 4.3.1 of [RFC1035].)
Iterative resolution: A name server may be presented with a query
that can only be answered by some other server. The two general
approaches to dealing with this problem are "recursive", in which
the first server pursues the query on behalf of the client at
another server, and "iterative", in which the server refers the
client to another server and lets the client pursue the query
there. (See Section 2.3 of [RFC1034].)
Hoffman, et al. Expires January 17, 2019 [Page 16]

Internet-Draft DNS Terminology July 2018
In iterative resolution, the client repeatedly makes non-recursive
queries and follows referrals and/or aliases. The iterative
resolution algorithm is described in Section 5.3.3 of [RFC1034].
Full resolver: This term is used in [RFC1035], but it is not defined
there. RFC 1123 defines a "full-service resolver" that may or may
not be what was intended by "full resolver" in [RFC1035]. This
term is not properly defined in any RFC.
Full-service resolver: Section 6.1.3.1 of [RFC1123] defines this
term to mean a resolver that acts in recursive mode with a cache
(and meets other requirements).
Priming: "The act of finding the list of root servers from a
configuration that lists some or all of the purported IP addresses
of some or all of those root servers." (Quoted from [RFC8109],
Section 2.) In order to operate in recursive mode, a resolver
needs to know the address of at least one root server. Priming is
most often done from a configuration setting that contains a list
of authoritative servers for the root zone.
Root hints: "Operators who manage a DNS recursive resolver typically
need to configure a 'root hints file'. This file contains the
names and IP addresses of the authoritative name servers for the
root zone, so the software can bootstrap the DNS resolution
process. For many pieces of software, this list comes built into
the software." (Quoted from [IANA_RootFiles]) This file is often
used in priming.
Negative caching: "The storage of knowledge that something does not
exist, cannot give an answer, or does not give an answer."
(Quoted from [RFC2308], Section 1)
Authoritative server: "A server that knows the content of a DNS zone
from local knowledge, and thus can answer queries about that zone
without needing to query other servers." (Quoted from [RFC2182],
Section 2.) It is a system that responds to DNS queries with
information about zones for which it has been configured to answer
with the AA flag in the response header set to 1. It is a server
that has authority over one or more DNS zones. Note that it is
possible for an authoritative server to respond to a query without
the parent zone delegating authority to that server.
Authoritative servers also provide "referrals", usually to child
zones delegated from them; these referrals have the AA bit set to
0 and come with referral data in the Authority and (if needed) the
Additional sections.
Hoffman, et al. Expires January 17, 2019 [Page 17]

Internet-Draft DNS Terminology July 2018
Authoritative-only server: A name server that only serves
authoritative data and ignores requests for recursion. It will
"not normally generate any queries of its own. Instead, it
answers non-recursive queries from iterative resolvers looking for
information in zones it serves." (Quoted from [RFC4697],
Section 2.4) In this case, "ignores requests for recursion" means
"responds to requests for recursion with responses indicating that
recursion was not performed".
Zone transfer: The act of a client requesting a copy of a zone and
an authoritative server sending the needed information. (See
Section 7 for a description of zones.) There are two common
standard ways to do zone transfers: the AXFR ("Authoritative
Transfer") mechanism to copy the full zone (described in
[RFC5936], and the IXFR ("Incremental Transfer") mechanism to copy
only parts of the zone that have changed (described in [RFC1995]).
Many systems use non-standard methods for zone transfer outside
the DNS protocol.
Secondary server: "An authoritative server which uses zone transfer
to retrieve the zone" (Quoted from [RFC1996], Section 2.1).
Secondary servers are also discussed in [RFC1034]. [RFC2182]
describes secondary servers in more detail. Although early DNS
RFCs such as [RFC1996] referred to this as a "slave", the current
common usage has shifted to calling it a "secondary".
Slave server: See secondary server.
Primary server: "Any authoritative server configured to be the
source of zone transfer for one or more [secondary] servers"
(Quoted from [RFC1996], Section 2.1) or, more specifically, "an
authoritative server configured to be the source of AXFR or IXFR
data for one or more [secondary] servers" (Quoted from [RFC2136]).
Primary servers are also discussed in [RFC1034]. Although early
DNS RFCs such as [RFC1996] referred to this as a "master", the
current common usage has shifted to "primary".
Master server: See primary server.
Primary master: "The primary master is named in the zone's SOA MNAME
field and optionally by an NS RR". (Quoted from [RFC1996],
Section 2.1). [RFC2136] defines "primary master" as "Master
server at the root of the AXFR/IXFR dependency graph. The primary
master is named in the zone's SOA MNAME field and optionally by an
NS RR. There is by definition only one primary master server per
zone." The idea of a primary master is only used by [RFC2136],
and is considered archaic in other parts of the DNS.
Hoffman, et al. Expires January 17, 2019 [Page 18]

Internet-Draft DNS Terminology July 2018
The idea of a primary master is only used in [RFC1996] and
[RFC2136]. A modern interpretation of the term "primary master"
is a server that is both authoritative for a zone and that gets
its updates to the zone from configuration (such as a master file)
or from UPDATE transactions.
Stealth server: This is "like a slave server except not listed in an
NS RR for the zone." (Quoted from [RFC1996], Section 2.1)
Hidden master: A stealth server that is a primary server for zone
transfers. "In this arrangement, the master name server that
processes the updates is unavailable to general hosts on the
Internet; it is not listed in the NS RRset." (Quoted from
[RFC6781], Section 3.4.3). An earlier RFC, [RFC4641], said that
the hidden master's name "appears in the SOA RRs MNAME field",
although in some setups, the name does not appear at all in the
public DNS. A hidden master can also be a secondary server for
the zone itself.
Forwarding: The process of one server sending a DNS query with the
RD bit set to 1 to another server to resolve that query.
Forwarding is a function of a DNS resolver; it is different than
simply blindly relaying queries.
[RFC5625] does not give a specific definition for forwarding, but
describes in detail what features a system that forwards needs to
support. Systems that forward are sometimes called "DNS proxies",
but that term has not yet been defined (even in [RFC5625]).
Forwarder: Section 1 of [RFC2308] describes a forwarder as "a
nameserver used to resolve queries instead of directly using the
authoritative nameserver chain". [RFC2308] further says "The
forwarder typically either has better access to the internet, or
maintains a bigger cache which may be shared amongst many
resolvers." That definition appears to suggest that forwarders
normally only query authoritative servers. In current use,
however, forwarders often stand between stub resolvers and
recursive servers. [RFC2308] is silent on whether a forwarder is
iterative-only or can be a full-service resolver.
Policy-implementing resolver: A resolver acting in recursive mode
that changes some of the answers that it returns based on policy
criteria, such as to prevent access to malware sites or
objectionable content. In general, a stub resolver has no idea
whether upstream resolvers implement such policy or, if they do,
the exact policy about what changes will be made. In some cases,
the user of the stub resolver has selected the policy-implementing
resolver with the explicit intention of using it to implement the
Hoffman, et al. Expires January 17, 2019 [Page 19]

Internet-Draft DNS Terminology July 2018
policies. In other cases, policies are imposed without the user
of the stub resolver being informed.
Open resolver: A full-service resolver that accepts and processes
queries from any (or nearly any) client. This is sometimes also
called a "public resolver", although the term "public resolver" is
used more with open resolvers that are meant to be open, as
compared to the vast majority of open resolvers that are probably
misconfigured to be open. Open resolvers are discussed in
[RFC5358]
Split DNS: The terms "split DNS" and "split-horizon DNS" have long
been used in the DNS community without formal definition. In
general, they refer to situations in which DNS servers that are
authoritative for a particular set of domains provide partly or
completely different answers in those domains depending on the
source of the query. The effect of this is that a domain name
that is notionally globally unique nevertheless has different
meanings for different network users. This can sometimes be the
result of a "view" configuration, described below.
[RFC2775], Section 3.8 gives a related definition that is too
specific to be generally useful.
View: A configuration for a DNS server that allows it to provide
different answers depending on attributes of the query, such as
for "split DNS". Typically, views differ by the source IP address
of a query, but can also be based on the destination IP address,
the type of query (such as AXFR), whether it is recursive, and so
on. Views are often used to provide more names or different
addresses to queries from "inside" a protected network than to
those "outside" that network. Views are not a standardized part
of the DNS, but they are widely implemented in server software.
Passive DNS: A mechanism to collect DNS data by storing DNS
responses from name servers. Some of these systems also collect
the DNS queries associated with the responses, although doing so
raises some privacy concerns. Passive DNS databases can be used
to answer historical questions about DNS zones such as which
values were present at a given time in the past, or when a name
was spotted first. Passive DNS databases allow searching of the
stored records on keys other than just the name and type, such as
"find all names which have A records of a particular value".
Anycast: "The practice of making a particular service address
available in multiple, discrete, autonomous locations, such that
datagrams sent are routed to one of several available locations."
Hoffman, et al. Expires January 17, 2019 [Page 20]

Internet-Draft DNS Terminology July 2018
(Quoted from [RFC4786], Section 2) See [RFC4786] for more detail
on Anycast and other terms that are specific to its use.
Instance: "When anycast routing is used to allow more than one
server to have the same IP address, each one of those servers is
commonly referred to as an 'instance'." "An instance of a server,
such as a root server, is often referred to as an 'Anycast
instance'." (Quoted from [RSSAC026])
Privacy-enabling DNS server: "A DNS server that implements DNS over
TLS [RFC7858] and may optionally implement DNS over DTLS
[RFC8094]." (Quoted from [RFC8310], Section 2)
7. Zones
This section defines terms that are used when discussing zones that
are being served or retrieved.
Zone: "Authoritative information is organized into units called
'zones', and these zones can be automatically distributed to the
name servers which provide redundant service for the data in a
zone." (Quoted from [RFC1034], Section 2.4)
Child: "The entity on record that has the delegation of the domain
from the Parent." (Quoted from [RFC7344], Section 1.1)
Parent: "The domain in which the Child is registered." (Quoted from
[RFC7344], Section 1.1) Earlier, "parent name server" was defined
in [RFC0882] as "the name server that has authority over the place
in the domain name space that will hold the new domain". (Note
that [RFC0882] was obsoleted by [RFC1034] and [RFC1035].)
[RFC0819] also has some description of the relationship between
parents and children.
Origin:
(a) "The domain name that appears at the top of a zone (just below
the cut that separates the zone from its parent). The name of the
zone is the same as the name of the domain at the zone's origin."
(Quoted from [RFC2181], Section 6.) These days, this sense of
"origin" and "apex" (defined below) are often used
interchangeably.
(b) The domain name within which a given relative domain name
appears in zone files. Generally seen in the context of
"$ORIGIN", which is a control entry defined in [RFC1035],
Section 5.1, as part of the master file format. For example, if
Hoffman, et al. Expires January 17, 2019 [Page 21]

Internet-Draft DNS Terminology July 2018
the $ORIGIN is set to "example.org.", then a master file line for
"www" is in fact an entry for "www.example.org.".
Apex: The point in the tree at an owner of an SOA and corresponding
authoritative NS RRset. This is also called the "zone apex".
[RFC4033] defines it as "the name at the child's side of a zone
cut". The "apex" can usefully be thought of as a data-theoretic
description of a tree structure, and "origin" is the name of the
same concept when it is implemented in zone files. The
distinction is not always maintained in use, however, and one can
find uses that conflict subtly with this definition. [RFC1034]
uses the term "top node of the zone" as a synonym of "apex", but
that term is not widely used. These days, the first sense of
"origin" (above) and "apex" are often used interchangeably.
Zone cut: The delimitation point between two zones where the origin
of one of the zones is the child of the other zone.
"Zones are delimited by 'zone cuts'. Each zone cut separates a
'child' zone (below the cut) from a 'parent' zone (above the
cut)." (Quoted from [RFC2181], Section 6; note that this is
barely an ostensive definition.) Section 4.2 of [RFC1034] uses
"cuts" instead of "zone cut".
Delegation: The process by which a separate zone is created in the
name space beneath the apex of a given domain. Delegation happens
when an NS RRset is added in the parent zone for the child origin.
Delegation inherently happens at a zone cut. The term is also
commonly a noun: the new zone that is created by the act of
delegating.
Authoritative data: "All of the RRs attached to all of the nodes
from the top node of the zone down to leaf nodes or nodes above
cuts around the bottom edge of the zone." (Quoted from [RFC1034],
Section 4.2.1) Note that this definition might inadvertently also
cause any NS records that appear in the zone to be included, even
those that might not truly be authoritative because there are
identical NS RRs below the zone cut. This reveals the ambiguity
in the notion of authoritative data, because the parent-side NS
records authoritatively indicate the delegation, even though they
are not themselves authoritative data.
[RFC4033], Section 2, defines "Authoritative RRset" which is
related to authoritative data but has a more precise definition.
Lame delegation: "A lame delegations exists when a nameserver is
delegated responsibility for providing nameservice for a zone (via
NS records) but is not performing nameservice for that zone
Hoffman, et al. Expires January 17, 2019 [Page 22]

Internet-Draft DNS Terminology July 2018
(usually because it is not set up as a primary or secondary for
the zone)." (Quoted from [RFC1912], Section 2.8)
Another definition is that a lame delegation "happens when a name
server is listed in the NS records for some domain and in fact it
is not a server for that domain. Queries are thus sent to the
wrong servers, who don't know nothing (at least not as expected)
about the queried domain. Furthermore, sometimes these hosts (if
they exist!) don't even run name servers." (Quoted from
[RFC1713], Section 2.3)
Glue records: "[Resource records] which are not part of the
authoritative data [of the zone], and are address resource records
for the [name servers in subzones]. These RRs are only necessary
if the name server's name is 'below' the cut, and are only used as
part of a referral response." Without glue "we could be faced
with the situation where the NS RRs tell us that in order to learn
a name server's address, we should contact the server using the
address we wish to learn." (Definition from [RFC1034],
Section 4.2.1)
A later definition is that glue "includes any record in a zone
file that is not properly part of that zone, including nameserver
records of delegated sub-zones (NS records), address records that
accompany those NS records (A, AAAA, etc), and any other stray
data that might appear" ([RFC2181], Section 5.4.1). Although glue
is sometimes used today with this wider definition in mind, the
context surrounding the [RFC2181] definition suggests it is
intended to apply to the use of glue within the document itself
and not necessarily beyond.
Bailiwick: "In-bailiwick" is an adjective to describe a name server
whose name is either a subdomain of or (rarely) the same as the
origin of the zone that contains the delegation to the name
server. In-bailiwick name servers may have glue records in their
parent zone (using the first of the definitions of "glue records"
in the definition above). (The term "bailiwick" means the
district or territory where a bailiff or policeman has
jurisdiction.)
"In-bailiwick" names are divided into two type of name server
names: "in-domain" names and "sibling domain" names.
* In-domain: an adjective to describe a name server whose name is
either subordinate to or (rarely) the same as the owner name of
the NS resource records. An in-domain name server name MUST
have glue records or name resolution fails. For example, a
Hoffman, et al. Expires January 17, 2019 [Page 23]

Internet-Draft DNS Terminology July 2018
delegation for "child.example.com" may have "in-domain" name
server name "ns.child.example.com".
* Sibling domain: a name server's name that is either subordinate
to or (rarely) the same as the zone origin and not subordinate
to or the same as the owner name of the NS resource records.
Glue records for sibling domains are allowed, but not
necessary. For example, a delegation for "child.example.com"
in "example.com" zone may have "sibling" name server name
"ns.another.example.com".
"Out-of-bailiwick" is the antonym of in-bailiwick. An adjective
to describe a name server whose name is not subordinate to or the
same as the zone origin. Glue records for out-of-bailiwick name
servers are useless. Following table shows examples of delegation
types.
Delegation |Parent|Name Server Name | Type
-----------+------+------------------+-----------------------------
com | . |a.gtld-servers.net|in-bailiwick / sibling domain
net | . |a.gtld-servers.net|in-bailiwick / in-domain
example.org| org |ns.example.org |in-bailiwick / in-domain
example.org| org |ns.ietf.org |in-bailiwick / sibling domain
example.org| org |ns.example.com |out-of-bailiwick
example.jp | jp |ns.example.jp |in-bailiwick / in-domain
example.jp | jp |ns.example.ne.jp |in-bailiwick / sibling domain
example.jp | jp |ns.example.com |out-of-bailiwick
Root zone: The zone of a DNS-based tree whose apex is the zero-
length label. Also sometimes called "the DNS root".
Empty non-terminals (ENT): "Domain names that own no resource
records but have subdomains that do." (Quoted from [RFC4592],
Section 2.2.2.) A typical example is in SRV records: in the name
"_sip._tcp.example.com", it is likely that "_tcp.example.com" has
no RRsets, but that "_sip._tcp.example.com" has (at least) an SRV
RRset.
Delegation-centric zone: A zone that consists mostly of delegations
to child zones. This term is used in contrast to a zone that
might have some delegations to child zones, but also has many data
resource records for the zone itself and/or for child zones. The
term is used in [RFC4956] and [RFC5155], but is not defined there.
Occluded name: "The addition of a delegation point via dynamic
update will render all subordinate domain names to be in a limbo,
still part of the zone, but not available to the lookup process.
The addition of a DNAME resource record has the same impact. The
Hoffman, et al. Expires January 17, 2019 [Page 24]

Internet-Draft DNS Terminology July 2018
subordinate names are said to be 'occluded'." (Quoted from
[RFC5936], Section 3.5)
Fast flux DNS: This "occurs when a domain is found in DNS using A
records to multiple IP addresses, each of which has a very short
Time-to-Live (TTL) value associated with it. This means that the
domain resolves to varying IP addresses over a short period of
time." (Quoted from [RFC6561], Section 1.1.5, with typo
corrected) In addition to having legitimate uses, fast flux DNS
can used to deliver malware. Because the addresses change so
rapidly, it is difficult to ascertain all the hosts. It should be
noted that the technique also works with AAAA records, but such
use is not frequently observed on the Internet as of this writing.
Reverse DNS, reverse lookup: "The process of mapping an address to a
name is generally known as a 'reverse lookup', and the IN-
ADDR.ARPA and IP6.ARPA zones are said to support the 'reverse
DNS'." (Quoted from [RFC5855], Section 1)
Forward lookup: "Hostname-to-address translation". (Quoted from
[RFC2133], Section 6)
arpa: Address and Routing Parameter Area Domain: "The 'arpa' domain
was originally established as part of the initial deployment of
the DNS, to provide a transition mechanism from the Host Tables
that were common in the ARPANET, as well as a home for the IPv4
reverse mapping domain. During 2000, the abbreviation was
redesignated to 'Address and Routing Parameter Area' in the hope
of reducing confusion with the earlier network name." (Quoted
from [RFC3172], Section 2.) .arpa is an "infrastructure domain", a
domain whose "role is to support the operating infrastructure of
the Internet". (Quoted from [RFC3172], Section 2.) See [RFC3172]
for more history of this name.
Service name: "Service names are the unique key in the Service Name
and Transport Protocol Port Number registry. This unique symbolic
name for a service may also be used for other purposes, such as in
DNS SRV records." (Quoted from [RFC6335], Section 5.)
8. Wildcards
Wildcard: [RFC1034] defined "wildcard", but in a way that turned out
to be confusing to implementers. For an extended discussion of
wildcards, including clearer definitions, see [RFC4592]. Special
treatment is given to RRs with owner names starting with the label
"*". "Such RRs are called 'wildcards'. Wildcard RRs can be
thought of as instructions for synthesizing RRs." (Quoted from
[RFC1034], Section 4.3.3)
Hoffman, et al. Expires January 17, 2019 [Page 25]

Internet-Draft DNS Terminology July 2018
Asterisk label: "The first octet is the normal label type and length
for a 1-octet-long label, and the second octet is the ASCII
representation for the '*' character. A descriptive name of a
label equaling that value is an 'asterisk label'." (Quoted from
[RFC4592], Section 2.1.1)
Wildcard domain name: "A 'wildcard domain name' is defined by having
its initial (i.e., leftmost or least significant) label be
asterisk label." (Quoted from [RFC4592], Section 2.1.1)
Closest encloser: "The longest existing ancestor of a name."
(Quoted from [RFC5155], Section 1.3) An earlier definition is "The
node in the zone's tree of existing domain names that has the most
labels matching the query name (consecutively, counting from the
root label downward). Each match is a 'label match' and the order
of the labels is the same." (Quoted from [RFC4592],
Section 3.3.1)
Closest provable encloser: "The longest ancestor of a name that can
be proven to exist. Note that this is only different from the
closest encloser in an Opt-Out zone." (Quoted from [RFC5155],
Section 1.3)
Next closer name: "The name one label longer than the closest
provable encloser of a name." (Quoted from [RFC5155],
Section 1.3)
Source of Synthesis: "The source of synthesis is defined in the
context of a query process as that wildcard domain name
immediately descending from the closest encloser, provided that
this wildcard domain name exists. 'Immediately descending' means
that the source of synthesis has a name of the form: <asterisk
label>.<closest encloser>." (Quoted from [RFC4592],
Section 3.3.1)
9. Registration Model
Registry: The administrative operation of a zone that allows
registration of names within that zone. People often use this
term to refer only to those organizations that perform
registration in large delegation-centric zones (such as TLDs); but
formally, whoever decides what data goes into a zone is the
registry for that zone. This definition of "registry" is from a
DNS point of view; for some zones, the policies that determine
what can go in the zone are decided by superior zones and not the
registry operator.
Hoffman, et al. Expires January 17, 2019 [Page 26]

Internet-Draft DNS Terminology July 2018
Registrant: An individual or organization on whose behalf a name in
a zone is registered by the registry. In many zones, the registry
and the registrant may be the same entity, but in TLDs they often
are not.
Registrar: A service provider that acts as a go-between for
registrants and registries. Not all registrations require a
registrar, though it is common to have registrars involved in
registrations in TLDs.
EPP: The Extensible Provisioning Protocol (EPP), which is commonly
used for communication of registration information between
registries and registrars. EPP is defined in [RFC5730].
WHOIS: A protocol specified in [RFC3912], often used for querying
registry databases. WHOIS data is frequently used to associate
registration data (such as zone management contacts) with domain
names. The term "WHOIS data" is often used as a synonym for the
registry database, even though that database may be served by
different protocols, particularly RDAP. The WHOIS protocol is
also used with IP address registry data.
RDAP: The Registration Data Access Protocol, defined in [RFC7480],
[RFC7481], [RFC7482], [RFC7483], [RFC7484], and [RFC7485]. The
RDAP protocol and data format are meant as a replacement for
WHOIS.
DNS operator: An entity responsible for running DNS servers. For a
zone's authoritative servers, the registrant may act as their own
DNS operator, or their registrar may do it on their behalf, or
they may use a third-party operator. For some zones, the registry
function is performed by the DNS operator plus other entities who
decide about the allowed contents of the zone.
Public suffix: "A domain that is controlled by a public registry."
(Quoted from [RFC6265], Section 5.3) A common definition for this
term is a domain under which subdomains can be registered by third
parties, and on which HTTP cookies (which are described in detail
in [RFC6265]) should not be set. There is no indication in a
domain name whether it is a public suffix; that can only be
determined by outside means. In fact, both a domain and a
subdomain of that domain can be public suffixes.
There is nothing inherent in a domain name to indicate whether it
is a public suffix. One resource for identifying public suffixes
is the Public Suffix List (PSL) maintained by Mozilla
(http://publicsuffix.org/).
Hoffman, et al. Expires January 17, 2019 [Page 27]

Internet-Draft DNS Terminology July 2018
For example, at the time this document is published, the "com.au"
domain is listed as a public suffix in the PSL. (Note that this
example might change in the future.)
Note that the term "public suffix" is controversial in the DNS
community for many reasons, and may be significantly changed in
the future. One example of the difficulty of calling a domain a
public suffix is that designation can change over time as the
registration policy for the zone changes, such as was the case
with the "uk" TLD in 2014.
Subordinate and Superordinate: These terms are introduced in
[RFC3731] for use in the registration model, but not defined
there. Instead, they are given in examples. "For example, domain
name 'example.com' has a superordinate relationship to host name
ns1.example.com'." "For example, host ns1.example1.com is a
subordinate host of domain example1.com, but it is a not a
subordinate host of domain example2.com." (Quoted from [RFC3731],
Section 1.1.) These terms are strictly ways of referring to the
relationship standing of two domains where one is a subdomain of
the other.
10. General DNSSEC
Most DNSSEC terms are defined in [RFC4033], [RFC4034], [RFC4035], and
[RFC5155]. The terms that have caused confusion in the DNS community
are highlighted here.
DNSSEC-aware and DNSSEC-unaware: These two terms, which are used in
some RFCs, have not been formally defined. However, Section 2 of
[RFC4033] defines many types of resolvers and validators,
including "non-validating security-aware stub resolver", "non-
validating stub resolver", "security-aware name server",
"security-aware recursive name server", "security-aware resolver",
"security-aware stub resolver", and "security-oblivious
'anything'". (Note that the term "validating resolver", which is
used in some places in DNSSEC-related documents, is also not
defined in those RFCs, but is defined below.)
Signed zone: "A zone whose RRsets are signed and that contains
properly constructed DNSKEY, Resource Record Signature (RRSIG),
Next Secure (NSEC), and (optionally) DS records." (Quoted from
[RFC4033], Section 2.) It has been noted in other contexts that
the zone itself is not really signed, but all the relevant RRsets
in the zone are signed. Nevertheless, if a zone that should be
signed contains any RRsets that are not signed (or opted out),
those RRsets will be treated as bogus, so the whole zone needs to
be handled in some way.
Hoffman, et al. Expires January 17, 2019 [Page 28]

Internet-Draft DNS Terminology July 2018
It should also be noted that, since the publication of [RFC6840],
NSEC records are no longer required for signed zones: a signed
zone might include NSEC3 records instead. [RFC7129] provides
additional background commentary and some context for the NSEC and
NSEC3 mechanisms used by DNSSEC to provide authenticated denial-
of-existence responses. NSEC and NSEC3 are described below.
Unsigned zone: Section 2 of [RFC4033] defines this as "a zone that
is not signed". Section 2 of [RFC4035] defines this as "A zone
that does not include these records [properly constructed DNSKEY,
Resource Record Signature (RRSIG), Next Secure (NSEC), and
(optionally) DS records] according to the rules in this section".
There is an important note at the end of Section 5.2 of [RFC4035]
that defines an additional situation in which a zone is considered
unsigned: "If the resolver does not support any of the algorithms
listed in an authenticated DS RRset, then the resolver will not be
able to verify the authentication path to the child zone. In this
case, the resolver SHOULD treat the child zone as if it were
unsigned."
NSEC: "The NSEC record allows a security-aware resolver to
authenticate a negative reply for either name or type non-
existence with the same mechanisms used to authenticate other DNS
replies." (Quoted from [RFC4033], Section 3.2.) In short, an
NSEC record provides authenticated denial of existence.
"The NSEC resource record lists two separate things: the next
owner name (in the canonical ordering of the zone) that contains
authoritative data or a delegation point NS RRset, and the set of
RR types present at the NSEC RR's owner name." (Quoted from
Section 4 of RFC 4034)
NSEC3: Like the NSEC record, the NSEC3 record also provides
authenticated denial of existence; however, NSEC3 records mitigate
against zone enumeration and support Opt-Out. NSEC3 resource
records require associated NSEC3PARAM resource records. NSEC3 and
NSEC3PARAM resource records are defined in [RFC5155].
Note that [RFC6840] says that [RFC5155] "is now considered part of
the DNS Security Document Family as described by Section 10 of
[RFC4033]". This means that some of the definitions from earlier
RFCs that only talk about NSEC records should probably be
considered to be talking about both NSEC and NSEC3.
Opt-out: "The Opt-Out Flag indicates whether this NSEC3 RR may cover
unsigned delegations." (Quoted from [RFC5155], Section 3.1.2.1.)
Opt-out tackles the high costs of securing a delegation to an
insecure zone. When using Opt-Out, names that are an insecure
Hoffman, et al. Expires January 17, 2019 [Page 29]

Internet-Draft DNS Terminology July 2018
delegation (and empty non-terminals that are only derived from
insecure delegations) don't require an NSEC3 record or its
corresponding RRSIG records. Opt-Out NSEC3 records are not able
to prove or deny the existence of the insecure delegations.
(Adapted from [RFC7129], Section 5.1)
Insecure delegation: "A signed name containing a delegation (NS
RRset), but lacking a DS RRset, signifying a delegation to an
unsigned subzone." (Quoted from [RFC4956], Section 2.)
Zone enumeration: "The practice of discovering the full content of a
zone via successive queries." (Quoted from [RFC5155],
Section 1.3.) This is also sometimes called "zone walking". Zone
enumeration is different from zone content guessing where the
guesser uses a large dictionary of possible labels and sends
successive queries for them, or matches the contents of NSEC3
records against such a dictionary.
Validation: Validation, in the context of DNSSEC, refers to one of
the following:
* Checking the validity of DNSSEC signatures
* Checking the validity of DNS responses, such as those including
authenticated denial of existence
* Building an authentication chain from a trust anchor to a DNS
response or individual DNS RRsets in a response
The first two definitions above consider only the validity of
individual DNSSEC components such as the RRSIG validity or NSEC
proof validity. The third definition considers the components of
the entire DNSSEC authentication chain, and thus requires
"configured knowledge of at least one authenticated DNSKEY or DS
RR" (as described in [RFC4035], Section 5).
[RFC4033], Section 2, says that a "Validating Security-Aware Stub
Resolver... performs signature validation" and uses a trust anchor
"as a starting point for building the authentication chain to a
signed DNS response", and thus uses the first and third
definitions above. The process of validating an RRSIG resource
record is described in [RFC4035], Section 5.3.
[RFC5155] refers to validating responses throughout the document,
in the context of hashed authenticated denial of existence; this
uses the second definition above.
Hoffman, et al. Expires January 17, 2019 [Page 30]

Internet-Draft DNS Terminology July 2018
The term "authentication" is used interchangeably with
"validation", in the sense of the third definition above.
[RFC4033], Section 2, describes the chain linking trust anchor to
DNS data as the "authentication chain". A response is considered
to be authentic if "all RRsets in the Answer and Authority
sections of the response [are considered] to be authentic"
([RFC4035]). DNS data or responses deemed to be authentic or
validated have a security status of "secure" ([RFC4035],
Section 4.3; [RFC4033], Section 5). "Authenticating both DNS keys
and data is a matter of local policy, which may extend or even
override the [DNSSEC] protocol extensions" ([RFC4033],
Section 3.1).
The term "verification", when used, is usually synonym for
"validation".
Validating resolver: A security-aware recursive name server,
security-aware resolver, or security-aware stub resolver that is
applying at least one of the definitions of validation (above), as
appropriate to the resolution context. For the same reason that
the generic term "resolver" is sometimes ambiguous and needs to be
evaluated in context (see Section 6), "validating resolver" is a
context-sensitive term.
Key signing key (KSK): DNSSEC keys that "only sign the apex DNSKEY
RRset in a zone."(Quoted from [RFC6781], Section 3.1)
Zone signing key (ZSK): "DNSSEC keys that can be used to sign all
the RRsets in a zone that require signatures, other than the apex
DNSKEY RRset." (Quoted from [RFC6781], Section 3.1) Also note
that a ZSK is sometimes used to sign the apex DNSKEY RRset.
Combined signing key (CSK): "In cases where the differentiation
between the KSK and ZSK is not made, i.e., where keys have the
role of both KSK and ZSK, we talk about a Single-Type Signing
Scheme." (Quoted from [RFC6781], Section 3.1) This is sometimes
called a "combined signing key" or CSK. It is operational
practice, not protocol, that determines whether a particular key
is a ZSK, a KSK, or a CSK.
Secure Entry Point (SEP): A flag in the DNSKEY RDATA that "can be
used to distinguish between keys that are intended to be used as
the secure entry point into the zone when building chains of
trust, i.e., they are (to be) pointed to by parental DS RRs or
configured as a trust anchor. Therefore, it is suggested that the
SEP flag be set on keys that are used as KSKs and not on keys that
are used as ZSKs, while in those cases where a distinction between
a KSK and ZSK is not made (i.e., for a Single-Type Signing
Hoffman, et al. Expires January 17, 2019 [Page 31]

Internet-Draft DNS Terminology July 2018
Scheme), it is suggested that the SEP flag be set on all keys."
(Quoted from [RFC6781], Section 3.2.3.) Note that the SEP flag is
only a hint, and its presence or absence may not be used to
disqualify a given DNSKEY RR from use as a KSK or ZSK during
validation.
The original definition of SEPs was in [RFC3757]. That definition
clearly indicated that the SEP was a key, not just a bit in the
key. The abstract of [RFC3757] says: "With the Delegation Signer
(DS) resource record (RR), the concept of a public key acting as a
secure entry point (SEP) has been introduced. During exchanges of
public keys with the parent there is a need to differentiate SEP
keys from other public keys in the Domain Name System KEY (DNSKEY)
resource record set. A flag bit in the DNSKEY RR is defined to
indicate that DNSKEY is to be used as a SEP." That definition of
the SEP as a key was made obsolete by [RFC4034], and the
definition from [RFC6781] is consistent with [RFC4034].
Trust anchor: "A configured DNSKEY RR or DS RR hash of a DNSKEY RR.
A validating security-aware resolver uses this public key or hash
as a starting point for building the authentication chain to a
signed DNS response. In general, a validating resolver will have
to obtain the initial values of its trust anchors via some secure
or trusted means outside the DNS protocol." (Quoted from
[RFC4033], Section 2)
DNSSEC Policy (DP): A statement that "sets forth the security
requirements and standards to be implemented for a DNSSEC-signed
zone." (Quoted from [RFC6841], Section 2)
DNSSEC Practice Statement (DPS): "A practices disclosure document
that may support and be a supplemental document to the DNSSEC
Policy (if such exists), and it states how the management of a
given zone implements procedures and controls at a high level."
(Quoted from [RFC6841], Section 2)
Hardware security module (HSM): A specialized piece of hardware that
is used to create keys for signatures and to sign messages. In
DNSSEC, HSMs are often used to hold the private keys for KSKs and
ZSKs and to create the signatures used in RRSIG records at
periodic intervals.
Signing software: Authoritative DNS servers that support DNSSEC
often contain software that facilitates the creation and
maintenance of DNSSEC signatures in zones. There is also stand-
alone software that can be used to sign a zone regardless of
whether the authoritative server itself supports signing.
Hoffman, et al. Expires January 17, 2019 [Page 32]

Internet-Draft DNS Terminology July 2018
Sometimes signing software can support particular HSMs as part of
the signing process.
11. DNSSEC States
A validating resolver can determine that a response is in one of four
states: secure, insecure, bogus, or indeterminate. These states are
defined in [RFC4033] and [RFC4035], although the two definitions
differ a bit. This document makes no effort to reconcile the two
definitions, and takes no position as to whether they need to be
reconciled.
Section 5 of [RFC4033] says:
A validating resolver can determine the following 4 states:
Secure: The validating resolver has a trust anchor, has a chain
of trust, and is able to verify all the signatures in the
response.
Insecure: The validating resolver has a trust anchor, a chain
of trust, and, at some delegation point, signed proof of the
non-existence of a DS record. This indicates that subsequent
branches in the tree are provably insecure. A validating
resolver may have a local policy to mark parts of the domain
space as insecure.
Bogus: The validating resolver has a trust anchor and a secure
delegation indicating that subsidiary data is signed, but
the response fails to validate for some reason: missing
signatures, expired signatures, signatures with unsupported
algorithms, data missing that the relevant NSEC RR says
should be present, and so forth.
Indeterminate: There is no trust anchor that would indicate that a
specific portion of the tree is secure. This is the default
operation mode.
Section 4.3 of [RFC4035] says:
Hoffman, et al. Expires January 17, 2019 [Page 33]

Internet-Draft DNS Terminology July 2018
A security-aware resolver must be able to distinguish between four
cases:
Secure: An RRset for which the resolver is able to build a chain
of signed DNSKEY and DS RRs from a trusted security anchor to
the RRset. In this case, the RRset should be signed and is
subject to signature validation, as described above.
Insecure: An RRset for which the resolver knows that it has no
chain of signed DNSKEY and DS RRs from any trusted starting
point to the RRset. This can occur when the target RRset lies
in an unsigned zone or in a descendent [sic] of an unsigned
zone. In this case, the RRset may or may not be signed, but
the resolver will not be able to verify the signature.
Bogus: An RRset for which the resolver believes that it ought to
be able to establish a chain of trust but for which it is
unable to do so, either due to signatures that for some reason
fail to validate or due to missing data that the relevant
DNSSEC RRs indicate should be present. This case may indicate
an attack but may also indicate a configuration error or some
form of data corruption.
Indeterminate: An RRset for which the resolver is not able to
determine whether the RRset should be signed, as the resolver
is not able to obtain the necessary DNSSEC RRs. This can occur
when the security-aware resolver is not able to contact
security-aware name servers for the relevant zones.
12. Security Considerations
These definitions do not change any security considerations for the
DNS.
13. IANA Considerations
None.
14. References14.1. Normative References
[IANA_RootFiles]
Internet Assigned Numbers Authority, "IANA Root Files",
2016, <http://www.iana.org/domains/root/files>.
Hoffman, et al. Expires January 17, 2019 [Page 34]